Portable public address system



Oct. 23, 1951 R. H. VOIGT 2,572,221

PORTABLE PUBLIC ADDRESS SYSTEM Filed Sept. 50, 1949 IN V EN TOR. B03527 l/O/ G 7} A OE/VEYS.

Patented Oct. 23, 1951 PORTABLE PUBLIC ADDRESS SYSTEM Robert H. Voig't, Detroit, Mich.

Application September so, 1949,, Serial No. 118,900

6 Claims.

This invention relates to public address {systems, and more particularly to a portable public address system particularly adapted to be employed on a vehicle such as a bus, or the like.

A main object of the invention is to provide a novel and improved portable public address system adapted for use either inside a passenger vehicle, such as a bus or the like, or outside the vehicle, said public address system involving'very simple components, being easy to install, and being ver reliable in operation.

A further object of theinvention is to provide an improved public address system of the type employing a carbon microphone in conjunction with an amplifier and one or more-loudspeakers,

the improved systeminvolvingvery inexpensive components, requiring only 'a small amount of battery power, and being readily adaptable for with the public address system illustrated in Figure 1.

Referring to the drawings, and'more particularly to Figure -1, I I designates a carbon microphone of conventional construction,preferably of the press-to-talk type. Designated at I2 is an input transformer having a primary winding I3 and a secondary Winding I4. Connected across secondary'windin'g I lis a lo'ad resistor l-5' which is of the order of 220,000 ohms. Designated at I is a potentiometer whose resistance isiof the order of l megohm. One terminal of thepotentiometer resistanceis connected to one terminal of load resistor I by a wire H. The otherterminal, shown at Il'yof the potentiometer resistance is connected througha coupling condenser I8 of the order of .006 mfd., tothe remaining terminal of the load resistor.

. The movable :tap of the, potentiometenf'shown at I9, isc'onnected' bya wire I9-to a'fixed contact I01, engageable by one poleAIM' ofiaithreepole,

' triple-throw switch, 58.

-' filament 32.

The pole I04 is connected b a wire I04 to the grid of an 1amplifier tube 2 I. Tube 2| has a cathode 22,'.;a plate 23 and a filament 24. Another fixed contact I06 of switch 58, engageable by pole I04, is connected by a wire I06 to the potentiometer terminal I1. Designated at I05 is still another fixed contact 'engageable by pole I04, as shown in .Figure .1, placing pole I04 in open circuit position. When pole I04 engages contact I01, potentiometer 'tap I9 controls the input voltage applied to the grid 20, for example, when the .system drives speakers mounted inside a vehicle and maximum volume is not required. When pole I04 engages :contact I06, terminal I? is connected directly to grid .20, applying maximum input voltage to tube 12I, as where it is desired to drive outsidespeakers.

Designated at 25 is a ground wire. One terminal of microphone II is .connected to ground wire 25. The other terminal of the microphone is connected by a wire 26 to one of the terminals of the transformer primary I3. Designated :at 21 is an output tube having a control grid 28, *a screen grid 29, a plate 30, a cathode 3I anda Cathode 3I is connected toground wire 25 through the resistance winding :of a potentiometer 33,'which provides the grid bias voltage for tube 21. The remainingxterminalwf the'primary I3 of the input transformer .isiconnected through a'resistor I08 of about 470*0hms to the movable tap 34 of the potentiometer;33 by a wire 35, whereby the voltage drop between .tap 34 and ground wire 25 provides a source of direct current for the circuit of the carbon microphone II. A by-passeondenser 36 is connectedacross the resistance winding of potentiometer 33. :Said resistance winding is of the order of 300 ohms.

Cathode 22 of amplifier tube 2| is connected toground'wire 25 through a-biasing resistor 31 of the orderof 5,000 ohms. Resistor 31 is bypassed by acondenser 38.

A filter condenser 39, of the order of 50mfd. is connected between wire and ground wire 25, in the manner shown in Figure 1, to filter out fluctuations in the D. C. voltage appearing :between tap 34 and ground wire 25. Resistor, I08 also aids in suppressing such fluctuations.

Designated at 40 .is a plate supply terminal. Connected to terminal 40 by a wire 41 is;a:.wire 42. Connected to wire 42 is one terminal ofra'resistor 43 of the order of 2200 ohms. Connected between therespective terminals .of resistor 43 and-ground are the respective filter condensers sand 45, which-may have capacities oflloeand 20 mid respectively. Connected between the and 59 are mechanically coupled together. 51 is successively engageable with respective staplate 23 of tube 2I and the junction of resistor 43 and condenser 44 is a load resistor 46, of the order of 200,000 ohms. Plate 23 is connected to the grid 28 of output tube 21 through a coupling condenser of the order of .006 mid. Connected between grid 28 and ground wire 25 is a grid leak resistor 48, of the order of 500,000 ohms.

Connected between screen grid 29 and ground wire 25 is a resistor 49 of the order of 50,000 ohms. Connected across plate 30 and wire 42 is theprimary 50 of an output transformer 5I. Screen grid 29 is connected to Wire 42 through a resistor 52 of the order of 10,000 ohms. Connected across primary 50 is a condenser 53 of the order of .006 mm. The secondary 54 of the output transformer has one of its terminals grounded by a wire 55. The other terminal of said secondary is connected by a wire 56 to one of the poles 5! of the three-pole, triple-throw switch 58. Another pole of switch 57 is shown at 59. Poles I04, 51 Pole tionary contacts 50, GI and 62. Pole 59 i successively engageable with successive stationary contacts 68, 64 and 65.

One terminal of each of the filaments 24 and 32 is connected to the ground wire 25 by respective wires 60 and 61. The other terminals of the filaments are connected by respective wires 68 and 69 to a wire 10 connected to switch contact 54. Switch contact 04 is connected to switch contact 65 by a wire I I. Connected between wires 25 and I is a pilot lamp I2. Designated at I3 is a terminal which is connected by a Wire 14 to pole 59. Designated at 15 and I6 are additional terminals, connected by respective wires TI and 18 to the respective switch contacts 62 and 61.

Designated at T9 and 80 are additional terminals. Terminal I9 is grounded. Terminal 80 is connected by a wire 8! to the stationary contact 82 of a pivoted hook switch 83. Switch 83 is grounded and is biased into engagement with contact 82 by a spring 84. Microphone H is adapted to be hung on hook switch 83, and when so hung, disengages switch 82 from contact 82.

Designated generally at 85 is a. conventional vibrapack connected at 83 to the positive terminal of the storage battery 8?, which may be the storage battery of a vehicle in which the public address system i employed. The negative terminal of the battery 81 is grounded. From Figure 1 it will be seen that the positive terminal of battery 81 is connected to terminal I3 through a wire 88, a radio frequency choke 89 and a wire 89. The high voltage output terminal of the vibrapack, shown at 9|, is connected by a wire 92 through a radio frequency choke 93 to the terminal 40.

Suitable loud-speakers shown at 94 and 95 may be respectively connected between the respective terminals I and I6 and ground, as shown.

From Figure 1 it will be seen that the filaments 24 and 32 will be de-energized when the switch poles 51 and 59 are in their open-circuit positions, as shown. When the microphone II is hung in switch hook 83, the vibrapack will also be deenergized, since its ground connection will be open at contact 82. The speakers 94, 95 will also be de-energized.

, When the switch poles 59 and 51 are moved respectively into engagement with contacts 64 and BI, wire I0 is connected to wire 14 by pole 59, energizing filaments 24 and 32, and wire 56 is connected to wire I8 through switch pole 5! connecting speaker 85 to the secondary 54 of output transformer 5|. At the same time, pole I04 em gages contact I05, connecting grid 20 to terminal II. Similarly, when poles 59 and 51 are engaged respectively with contacts 65 and 62, the filaments are likewise energized and speaker 94 is connected to the secondary of output transformer 5I. At this time, pole I04 engages contact I01, connecting grid 20 to potentiometer tap I9.

With the switch poles 59 and 5! in either of the above-described, circuit-closing positions, when microphone I I is lifted from hook switch 83, the

vibrapack becomes energized by the completion of its ground circuit at contact 82, and plate volt age is provided at terminal 40. The microphone circuit is provided with current by its connection between tap 34 and wire 25. The fluctuations of sound energy impinging on the diaphragm of the microphone I I produces expansion and compression of the carbon granules therein, causing corresponding variations in the current in primary I3, and producing an audio signal across the load resistor I5. This signal is amplified in tube 2I and applied to the grid 28 of the output tube 21. The output of tube 2'! is delivered to the speaker 94 or through the output transformer 5!.

It will be noted that by connecting the carbon microphone II in the manner shown in Figure 1, no additional battery is required to energize the microphone, the microphone current being derived from the cathode resistor 33 of the output tube 21.

Instead of employing a vibrapack, a dynamotor 96 may be connected to the storage battery 81 and to the terminals I9, 80, 48 and T3 in the manner illustrated in Figure 3. The highvoltage commutator of the dynamotor is shown at 91, and the high-voltage circuit is controlled by the hook switch 83 in the same manner as in Figure 1. The low-voltage commutator of the dynamotor is shown at 98.

The public address system may be powered from a -volt alternating current power supply by wiring the filaments 24, 32, pilot light I2, a suitable dropping resistor 99, and a switch I00 in series across the power lines and employing a half-wave rectifier IOI in the well-known manner illustrated in Figure 2 to provide the required filament and plate current for the amplifier. The connections to terminals 80 and 40 in Figure 2 are the same as those shown in Figure 1. A suitable shunt resistor I03 is employed across the pilot light I2 to prevent excessive voltage from being applied across said pilot light.

The public address system schematically illustrated in Figure 1 is especially suitable for use as a compact, versatile and economical unit to be mounted in a school bus or similar passenger vehicle for providing a means of furnishing safety instructions or other information to the passengers of the vehicle.

The switch 58 enables either the speaker 94, mounted inside the vehicle. or the speaker 95, mounted at an outside location, to be selected and automatically applies maximum input voltage to amplifier tube 2I when the outside speaker 95 is selected by connecting terminal ll of the potentiometer winding IE to the grid 20 of tube 2 I.

The microphone II may be mounted in the vehicle in any suitable manner, for example, it may be carried on a flexible shaft attached to the side of the vehicle near the driver so that it may be moved into position before the driver when required. A floor switch may be substituted for the hook switch 83.

If increased power output is desired, an additional output tube may be connected in parallel with or in any other suitable arrangement with relation to output tube 21.

Although a specific embodiment of an improved public address system has been disclosed in the foregoing description, it will be understood that various modifications within the spirit of the invention may occur to those skilled in the art. Therefore, it is intended that no limitations be placed on the invention except as defined by the scope of the appended claims.

What is claimed is:

1. In a public address system, a carbon microphone, a vacuum tube amplifier including at least one tube having a grid, a plate, and a cathode, a cathode resistor, and means for connecting said cathode resistor to provide a bias voltage for the grid, an input circuit connected to said carbon microphone and coupled to said amplifier, and circuit means connecting at least a portion of said cathode resistor, said input circuit and said microphone in series and arranged to energize said microphone.

2. In a public address system, a carbon microphone, a vacuum tube amplifier including at least one tube having a grid, a plate, and a cathode, a cathode resistor, and means for connecting said cathode resistor to provide a bias voltage for the grid, a tap movably contacting said cathode resistor, an input circuit connected in series with said carbon microphone and coupled to said amplifier, and circuit means connecting said input circuit and microphone between said tap and one of the terminals of said cathode resistor, whereby at least a portion of said cathode resistor is in series with the microphone and input circuit and is arranged to energize said microphone.

3. In a public address system, a carbon microphone, a vacuum tube amplifier including at least one tube having a grid, a plate, and a cathode, a cathode resistor, and means for connecting said cathode resistor to provide a bias voltage for the grid, a tap movably contacting said cathode resistor, an input circuit including a transformer, the carbon microphone bein connected in series with the primary of said transformer and the input circuit being coupled to said amplifier, and circuit means connecting said carbon microphone and primary in series between said tap and one of the terminals of said cathode resistor, whereby at least a portion of said resistor is in series with the primary and microphone and is arranged to energize said microphone.

4. In a public address system, a carbon microphone, a vacuum tube amplifier including at least one tube having a grid, a plate, and a cathode, a cathode resistor, and means for connecting said cathode resistor to provide a bias voltage for the grid, an input circuit connected to said carbon microphone and coupled to said amplifier, circuit means connecting said cathode resistor in series with said input circuit and microphone and arranged to energize said microphone, a plurality of output devices, switch means for selectively connecting said output devices to the output circuit of the amplifier, and means controlling the input to s-aid'amplifier in accordance with the operation of said switch means.

5. In a public address system, a carbon microhone, a vacuum tube amplifier including at least one tube having a grid, a plate, and a cathode, a cathode resistor, and means for connecting said cathode resistor to provide a bias voltage for the grid, a power source, circuit means connecting said power source to the amplifier, an input circuit connected in series with said carbon microphone and coupled to said amplifier, circuit means connecting at least a portion of said cathode resistor in series with said input circuit and microphone and arranged to energize said microphone, a plurality of output devices, movable switch means for selectively connecting said output devices to the output circuit of the amplifier, said switch means having an open circuit position and being formed and arranged to uncouple the input circuit from the amplifier when moved into said open circuit position.

6. In a public address system, a carbon microphone, a vacuum tube amplifier including at least one tube having a grid, a plate, and a cathode, a cathode resistor, and means for connecting said cathode resistor to provide a bias voltage for the rid, an input circuit connected in series with means, whereby difierent signal levels may be furnished to the amplifier for amplification in accordance with the selection of different output devices.

ROBERT H. VOIGT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

